Galactic Molecular Clouds Research Articles

Early high-resolution millimeter-wave maps from ToITEC

TolTEC is a polarization-sensitive camera at millimeter wavelengths with unprecedented sensitivity and 5-10 arcsecond resolution in three photometric bands. TolTEC achieved first light on the 50-meter Large Millimeter Telescope in July 2022, just prior to a planned summer telescope maintenance shutdown, and began commissioning observations when the telescope resumed observations in December 2022. The commissioning program consisted of observations of targeted nebulae, molecular clouds, polarized quasars, galaxies, and clusters of galaxies. The goals of these observations include demonstrating the science capabilities of the camera and characterizing the performance and instrumental properties, including noise, beams, and polarization sensitivity in its three bands centered at 1.1, 1.4, and 2.0 mm |with angular resolutions of 5, 6, and 10 arcseconds, respectively. We present early results from the commissioning observations of the Crab Nebula. Upcoming observations will include selected proposals from the broader astronomy community in Mexico and the United States, as well as legacy surveys focused on mapping galactic molecular clouds, cold dust emission in local galaxies, polarized dust emission in filaments around star-forming regions, massive galaxy clusters, and distant obscured star-forming galaxies.

The three-dimensional structure of galactic molecular cloud complexes out to 2.5 kpc

ABSTRACT Knowledge of the three-dimensional structure of Galactic molecular clouds is important for understanding how clouds are affected by processes such as turbulence and magnetic fields and how this structure affects star formation within them. Great progress has been made in this field with the arrival of the Gaia mission, which provides accurate distances to ∼109 stars. Combining these distances with extinctions inferred from optical–infrared, we recover the three-dimensional structure of 16 Galactic molecular cloud complexes at ∼1 pc resolution using our novel three-dimensional dust mapping algorithm dustribution. Using astrodendro, we derive a catalogue of physical parameters for each complex. We recover structures with aspect ratios between 1 and 11, i.e. everything from near-spherical to very elongated shapes. We find a large variation in cloud environments that is not apparent when studying them in two dimensions. For example, the nearby California and Orion A clouds look similar on sky, but we find California to be more sheet-like, and massive, which could explain their different star formation rates. By calculating the total mass of these individual clouds, we demonstrate that it is necessary to define cloud boundaries in three dimensions in order to obtain an accurate mass; simply integrating the extinction overestimates masses. We find that Larson’s relationship on mass versus radius holds true whether you assume a spherical shape for the cloud or take their true extents.

Depleted carbon isotope compositions observed at Gale crater, Mars

Obtaining carbon isotopic information for organic carbon from Martian sediments has long been a goal of planetary science, as it has the potential to elucidate the origin of such carbon and aspects of Martian carbon cycling. Carbon isotopic values (δ13CVPDB) of the methane released during pyrolysis of 24 powder samples at Gale crater, Mars, show a high degree of variation (-137 ± 8‰ to +22 ± 10‰) when measured by the tunable laser spectrometer portion of the Sample Analysis at Mars instrument suite during evolved gas analysis. Included in these data are 10 measured δ13C values less than -70‰ found for six different sampling locations, all potentially associated with a possible paleosurface. There are multiple plausible explanations for the anomalously depleted 13C observed in evolved methane, but no single explanation can be accepted without further research. Three possible explanations are the photolysis of biological methane released from the subsurface, photoreduction of atmospheric CO2, and deposition of cosmic dust during passage through a galactic molecular cloud. All three of these scenarios are unconventional, unlike processes common on Earth.

Interaction of HCO+ Cations With Interstellar Negative Grains. Quantum Chemical Investigation and Astrophysical Implications

In cold galactic molecular clouds, dust grains are coated by icy mantles and are prevalently charged negatively, because of the capture of the electrons in the gas. The interaction of the charged grains with gaseous cations is known to neutralize them. In this work, we focus on the chemical consequences of the neutralization process of HCO+, often the most abundant cation in molecular clouds. More specifically, by means of electronic structure calculations, we have characterized the energy and the structure of all possible product species once the HCO+ ion adsorbs on water clusters holding an extra electron. Two processes are possible: (i) electron transfer from the negative water cluster to the HCO+ ion or (ii) a proton transfer from HCO+ to the negative water cluster. Energetic considerations favor electron transfer. Assuming this scenario, two limiting cases have been considered in astrochemical models: (a) all the neutralized HCO+ is retained as neutral HCO adsorbed on the ice and (b) all the neutralized HCO+ gets desorbed to the gas phase as HCO. None of the two limiting cases appreciably contribute to the HCO abundance on the grain surfaces or in the gas.

Extended X-Ray Emission in Compton Thick AGN with Deep Chandra Observations

Abstract We present the spatial analysis of five Compton thick (CT) active galactic nuclei (AGNs), including MKN 573, NGC 1386, NGC 3393, NGC 5643, and NGC 7212, for which high-resolution Chandra observations are available. For each source, we find hard X-ray emission (>3 keV) extending to ∼kiloparsec scales along the ionization cone, and for some sources, in the cross-cone region. This collection represents the first, high-signal sample of CT AGN with extended hard X-ray emission for which we can begin to build a more complete picture of this new population of AGN. We investigate the energy dependence of the extended X-ray emission, including possible dependencies on host galaxy and AGN properties, and find a correlation between the excess emission and obscuration, suggesting a connection between the nuclear obscuring material and the galactic molecular clouds. Furthermore, we find that the soft X-ray emission extends farther than the hard X-rays along the ionization cone, which may be explained by a galactocentric radial dependence on the density of molecular clouds due to the orientation of the ionization cone with respect to the galactic disk. These results are consistent with other CT AGN with observed extended hard X-ray emission (e.g., ESO 428-G014 and the Ma et al. CT AGN sample), further demonstrating the ubiquity of extended hard X-ray emission in CT AGN.

Метод автоматичної побудови матриці похибок радіотелескопа РТ-32. Методика автоматичного оцінювання похибок наведення

On March 15th, 2021, scientists of the National Space Facilities Control and Tests Center and the Radio Astronomical Institute of the National Academy of Sciences of Ukraine carried out preliminary observations with the Ukrainian new generation radio telescope RT-32 (Zolochiv, Lviv region). The extragalactic radiation of radio galaxy 3C84 (Perseus-A), masers from the galactic molecular cloud W3, radio emission of methanol maser from the galactic radio source G188.946 + 0.886 were observed and successfully recorded. Observations were performed as training in the framework of preparation for the launch of a joint Ukrainian-Latvian radio astronomy project lzp-2020/2-0121. The results of the observations confirmed the world level of RT-32 radio telescope characteristics, the efficiency of the primary error matrix and revealed several shortcomings in the functioning of the tracking system. It was found that the primary tracking error matrix has insufficient discreteness and contains errors of the first and second types. In the article, we present a method of automatic construction of the radio telescope error matrix according to the data of a radiometric receiver and receivers-recorders. The method of construction provides automatic processing of the obtained radiometric data. The results of verification of the developed method using the reference radio sources of different types and the elements of tracking errors’ matrix by the elevation and azimuth obtained when using it are presented. The results obtained with the proposed method were included in the radio telescope control system and allowed us to increase the aiming accuracy of the RT-32 radio telescope.

Positron excess from cosmic ray interactions in galactic molecular clouds

The recent data on cosmic ray positron flux measured near the Earth by the Alpha Magnetic Spectrometer (AMS-02) experiment extends to TeV energy. The recently observed data by AMS-02 clearly confirms that the positron flux rises with energy and shows a peak near a few hundred GeV. This rising positron flux cannot be explained by interactions of cosmic rays with interstellar hydrogen gas. Pulsars, dark matter and many other innovative physical scenarios have been studied to explain this rising of the positron flux, also known as positron excess. In this paper, our goal is to study whether secondary production due to cosmic ray interactions in nearby Galactic Molecular Clouds (GMCs) can contribute significantly to the observed positron spectrum on Earth. Due to the progress in multi-wavelength astronomy, many new GMCs have been discovered in our Galaxy recently. Using large scale CO survey, 1064 GMCs were detected in the Galaxy, which reside in the Galactic plane. Alongside that, very recent survey implemented the optical/IR dust extinction measurements, to trace 567 GMCs within 4 kpc of Earth, also residing in the Galactic plane. We use the updated list of GMCs reported in recent papers, which are distributed in the Galactic plane, to find the secondary positrons produced in them in interactions of cosmic rays with molecular hydrogen. Moreover, by analyzing the Fermi-LAT data, new GMCs have been discovered near the Galactic plane. We also include some of these GMCs closest to the Earth where cosmic ray interactions are producing secondaries. It has been speculated earlier that cosmic rays may be reaccelerated in some GMCs. We select 7 GMCs out of 567 GMCs recently reported, within 4 kpc of Earth, where reacceleration due to magnetized turbulence is assumed. We include a small hardened component of secondary positrons, produced from interaction of reaccelerated CRs in those 7 GMCs. We use publicly available code DRAGON for our simulation setup to study CR propagation in the Galaxy and show that the observed positron spectrum can be well explained in the energy range of 1 to 1000 GeV by our self consistent model.

Nonrelativistic formation of scalar clumps as a candidate for dark matter

We propose a new mechanism for the formation of dark matter clumps in the radiation era. We assume that a light scalar field is decoupled from matter and oscillates harmonically around its vacuum expectation value. We include self-interactions and consider the nonrelativistic regime. The scalar dynamics are described by a fluid approach where the fluid pressure depends on both quantum and self-interaction effects. When the squared speed of sound of the scalar fluid becomes negative, an instability arises and the fluctuations of the scalar energy-density field start growing. They eventually become nonlinear and clumps form. Subsequently, the clumps aggregate and reach a universal regime. Afterwards, they play the role of cold dark matter. We apply this mechanism first to a model with a negative quartic term stabilised by a positive self-interaction of order six, and then to axion monodromy, where a subdominant cosine potential corrects a mass term. In the first case, the squared speed of sound becomes negative when the quartic term dominates, leading to a tachyonic instability. For axion monodromy, the instability starts very slowly after the squared speed of sound first becomes negative and then oscillates around zero. Initially the density perturbations perform acoustic oscillations due to the quantum pressure. Eventually, they start growing exponentially due to a parametric resonance. In both scenarios, the scalar-field clumps span a wide range of scales and masses, running from the size of atoms to that of galactic molecular clouds, and from $10^{-3} \, {\rm gram}$ to thousands of solar masses. Because of finite-size effects, both from the source and the lens, these dark matter clumps are far beyond the reach of microlensing observations.

ALMA resolves molecular clouds in metal-poor Magellanic Bridge A

Context. The Magellanic Bridge is a tidal feature located between the Magellanic Clouds, containing young stars formed in situ. Its proximity allows high-resolution studies of molecular gas, dust, and star formation in a tidal low-metallicity environment. Aims. Our goal is to characterize gas and dust emission in Magellanic Bridge A, the source with the highest 870 μm excess of emission found in single-dish surveys. Methods. Using the ALMA telescope including the Morita Array, we mapped a 3′ field of view centered on the Magellanic Bridge A molecular cloud, in 1.3 mm continuum emission and 12CO(2−1) line emission at subparsec resolution. This region was also mapped in continuum at 870 μm and in 12CO(2−1) line emission at ~6 pc resolution with the APEX telescope. To study its dust properties, we also use archival Herschel and Spitzer data. We combine the ALMA and APEX 12CO(2−1) line cubes to study the molecular gas emission. Results. Magellanic Bridge A breaks up into two distinct molecular clouds in dust and 12CO(2−1) emission, which we call North and South. Dust emission in the North source, according to our best parameters from fitting the far-infrared fluxes, is ≈3 K colder than in the South source in correspondence to its less developed star formation. Both dust sources present large submillimeter excesses in LABOCA data: according to our best fits the excess over the modified blackbody (MBB) fit to the Spitzer/Herschel continuum is E(870 μm) ~ 7 and E(870 μm) ~ 3 for the North and South sources, respectively. Nonetheless, we do not detect the corresponding 1.3 mm continuum with ALMA. Our limits are compatible with the extrapolation of the MBB fits, and therefore we cannot independently confirm the excess at this longer wavelength. The 12CO(2−1) emission is concentrated in two parsec-sized clouds with virial masses of around 400 and 700 M⊙. Their bulk volume densities are n(H2) ~ 0.7−2.6 × 103 cm−3, higher than typical bulk densities of Galactic molecular clouds. The 12CO luminosity to H2 mass conversion factor αCO is 6.5 and 15.3 M⊙ (K km s−1 pc2)−1 for the North and South clouds, calculated using their respective virial masses and 12CO(2−1) luminosities. Gas mass estimates from our MBB fits to dust emission yields masses M ~ 1.3 × 103 M⊙ and 2.9 × 103 M⊙ for North and South, respectively, a factor of ~4 higher than the virial masses we infer from 12CO.

Infrared Dark Clouds and High-mass Star Formation Activity in Galactic Molecular Clouds

Abstract Ever since their discovery, infrared dark clouds (IRDCs) are generally considered to be the sites just at the onset of high-mass (HM) star formation. In recent years, it has been realized that not all IRDCs harbor HM young stellar objects (YSOs). Only those IRDCs satisfying a certain mass–size criterion, or equivalently above a certain threshold density, are found to contain HMYSOs. In all cases, IRDCs provide ideal conditions for the formation of stellar clusters. In this paper, we study the massive stellar content of IRDCs to readdress the relation between IRDCs and HM star formation. For this purpose, we have identified all IRDCs associated with a sample of 12 Galactic molecular clouds (MCs). The selected MCs have been the target of a systematic search for YSOs in an earlier study. The cataloged positions of YSOs have been used to search all YSOs embedded in each identified IRDC. In total, we have found 834 YSOs in 128 IRDCs. The sample of IRDCs have mean surface densities of 319 M ⊙ pc − 2 , mean mass of 1062 M ⊙ , and a mass function power-law slope −1.8, which are similar to the corresponding properties for the full sample of IRDCs and resulting physical properties in previous studies. We find that all those IRDCs containing at least one intermediate to HM young star satisfy the often-used mass–size criterion for forming HM stars. However, not all IRDCs satisfying the mass–size criterion contain HM stars. We find that the often-used mass–size criterion corresponds to 35% probability of an IRDC forming a massive star. Twenty-five (20%) of the IRDCs are potential sites of stellar clusters of mass more than 100 M ⊙ .

Analysis of Galactic molecular cloud polarization maps: a review of the methods

The Davis-Chandrasekhar-Fermi (DCF) method using the Angular Dispersion Function (ADF), the Histogram of Relative Orientations (HROs) and the Polarization-Intensity Gradient Relation (P-IGR) are the most common tools used to analyse maps of linearly polarized emission by thermal dust grains at submilliter wavelengths in molecular clouds and star-forming regions. A short review of these methods is given. The combination of these methods will provide valuable tools to shed light on the impact of the magnetic fields on the formation and evolution of subparsec scale hub-filaments that will be mapped with the NIKA2 camera and future experiments.

The Density Gradient Inside Molecular-gas Clumps as a Booster of Their Star Formation Activity

Abstract Star-forming regions presenting a density gradient experience a higher star formation rate than if they were of uniform density. We refer to the ratio between the star formation rate of a spherical centrally concentrated gas clump and the star formation rate that this clump would experience if it were of uniform density as the magnification factor ζ. We map ζ as a function of clump mass, radius, initial volume-density profile, and star formation timespan. For clumps with a steep density profile (i.e., a power-law slope ranging from −3 to −4, as observed in some high-density regions of Galactic molecular clouds), we find their star formation rate to be at least an order of magnitude higher than their top-hat equivalent. This implies that such clumps experience faster and more efficient star formation than expected based on their mean freefall time. This also implies that measurements of the star formation efficiency per freefall time of clumps based on their global properties, namely, mass, mean volume density, and star formation rate, present wide fluctuations. These reflect the diversity in the density profile of star-forming clumps, not necessarily variations in the physics of star formation. Steep density profiles inside star-cluster progenitors may be instrumental in the formation of multiple stellar populations, such as those routinely observed in old globular clusters.

Northern Galactic Molecular Cloud Clumps in Hi-GAL: Clump and Star Formation within Clouds

Abstract We investigate how the properties of Galactic giant molecular clouds (GMCs) and their denser substructures (clumps) correlate with the local star formation rate (SFR). We trace clouds using the 12CO(3−2) transition, as observed by the CO High Resolution Survey. We identify their constituent clumps using thermal dust emission, as observed by the Herschel infrared GALactic plane survey. We estimate SFRs in these clouds using 70 μm emission. In total, we match 3674 clumps to 473 clouds in position–position–velocity space spanning the Galactic longitude range 10° < ℓ < 56°. We find that more massive clouds produce more clumps and more massive clumps. These clumps have average number densities an order of magnitude greater than their host clouds. We find a mean clump mass fraction of . This mass fraction weakly varies with mass and mass surface density of clouds, and shows no clear dependence on the virial parameter and line width of the clouds. The average clump mass fraction is only weakly dependent upon Galactocentric radius. Although the scatter in our measured properties is significant, the SFR for clouds is independent of clump mass fraction. However, there is a positive correlation between the depletion times for clouds and clump mass fraction. We find a star formation efficiency per freefall time of ϵ ff = 0.15% for GMCs but ϵ ff = 0.37% for clumps.

Revisiting the dust properties in the molecular clouds of the Large Magellanic Cloud

Context. Some Galactic molecular clouds show signs of dust evolution as compared to the diffuse interstellar medium, most of the time through indirect evidence such as color ratios, increased dust emissivity, or scattering (coreshine). These signs are not a feature of all Galactic clouds. Moreover, molecular clouds in the Large Magellanic Cloud (LMC) have been analyzed in a previous study based on Spitzer and IRIS data, at 4′ angular resolution, with the use of one single dust model, and did not show any signs of dust evolution. Aims. In this present analysis we investigate the dust properties associated with the different gas phases (including the ionized phase this time) of the LMC molecular clouds at 1′ angular resolution (four times greater than the previous analysis) and with a larger spectral coverage range thanks to Herschel data. We also ensure the robustness of our results in the framework of various dust models. Methods. We performed a decomposition of the dust emission in the infrared (from 3.6 to 500 μm) associated with the atomic, molecular, and ionized gas phases in the molecular clouds of the LMC. The resulting spectral energy distributions were fitted with four distinct dust models. We then analyzed the model parameters such as the intensity of the radiation field and the relative dust abundances, as well as the slope of the emission spectra at long wavelengths. Results. This work allows dust models to be compared with infrared data in various environments for the first time, which reveals important differences between the models at short wavelengths in terms of data fitting (mainly in the polycyclic aromatic hydrocarbon bands). In addition, this analysis points out distinct results according to the gas phases, such as dust composition directly affecting the dust temperature and the dust emissivity in the submillimeter and different dust emission in the near-infrared (NIR). Conclusions. We observe direct evidence of dust property evolution from the diffuse to the dense medium in a large sample of molecular clouds in the LMC. In addition, the differences in the dust component abundances between the gas phases could indicate different origins of grain formation. We also point out the presence of a NIR-continuum in all gas phases, with an enhancement in the ionized gas. We favor the hypothesis of an additional dust component as the carrier of this continuum.

A Systematic TMRT Observational Study of Galactic 12C/13C Ratios from Formaldehyde

Abstract We present observations of the C-band 110–111 (4.8 GHz) and Ku-band 211–212 (14.5 GHz) K-doublet lines of H2CO and the C-band 110–111 (4.6 GHz) line of H2 13CO toward a large sample of Galactic molecular clouds, through the Shanghai Tianma 65 m radio telescope (TMRT). Our sample with 112 sources includes strong H2CO sources from the TMRT molecular line survey at C-band and other known H2CO sources. All three lines are detected toward 38 objects (43 radial velocity components) yielding a detection rate of 34%. Complementary observations of their continuum emission at both C- and Ku-bands were performed. Combining spectral line parameters and continuum data, we calculate the column densities, the optical depths and the isotope ratio H2 12CO/H2 13CO for each source. To evaluate photon trapping caused by sometimes significant opacities in the main isotopologue’s rotational mm-wave lines connecting our measured K-doublets, and to obtain 12C/13C abundance ratios, we used the RADEX non-LTE model accounting for radiative transfer effects. This implied the use of the new collision rates from Wiesenfeld & Faure. Also implementing distance values from trigonometric parallax measurements for our sources, we obtain a linear fit of 12C/13C = (5.08 ± 1.10)D GC + (11.86 ± 6.60), with a correlation coefficient of 0.58. D GC refers to Galactocentric distances. Our 12C/13C ratios agree very well with the ones deduced from CN and C18O but are lower than those previously reported on the basis of H2CO, tending to suggest that the bulk of the H2CO in our sources was formed on dust grain mantles and not in the gas phase.

FOREST Unbiased Galactic Plane Imaging Survey with the Nobeyama 45 m telescope (FUGIN). V. Dense gas mass fraction of molecular gas in the Galactic plane

Abstract Recent observations of the nearby Galactic molecular clouds indicate that the dense gas in molecular clouds has quasi-universal properties on star formation, and observational studies of extra-galaxies have shown a galactic-scale correlation between the star formation rate (SFR) and the surface density of molecular gas. To reach a comprehensive understanding of both properties, it is important to quantify the fractional mass of dense gas in molecular clouds, fDG. In particular, for the Milky Way (MW) there are no previous studies resolving fDG disk over a scale of several kpc. In this study, fDG was measured over 5 kpc in the first quadrant of the MW, based on the CO J = 1–0 data in l = 10°–50° obtained as part of the FOREST Unbiased Galactic plane Imaging survey with the Nobeyama 45 m telescope (FUGIN) project. The total molecular mass was measured using 12CO, and the dense gas mass was estimated using C18O. The fractional masses, including fDG, in the region within ±30% of the distances to the tangential points of the Galactic rotation (e.g., the Galactic Bar, Far-3 kpc Arm, Norma Arm, Scutum Arm, Sagittarius Arm, and inter-arm regions) were measured. As a result, an averaged fDG of $2.9^{+2.6}_{-2.6}$% was obtained for the entirety of the target region. This low value suggests that dense gas formation is the primary factor in inefficient star formation in galaxies. It was also found that fDG shows large variations depending on the structures in the MW disk. In the Galactic arms, fDG was estimated to be ∼4%–5%, while in the bar and inter-arm regions it was as small as ∼0.1%–0.4%. These results indicate that the formation/destruction processes of the dense gas and their timescales are different for different regions in the MW, leading to differences in Star formation efficiencies.

Comparative study of gamma-ray emission from molecular clouds and star-forming galaxies

Star-forming regions on different scales, such as giant molecular clouds in our Galaxy and star-forming galaxies, emit GeV gamma-rays. These are thought to originate from hadronic interactions of cosmic-ray (CR) nuclei with the interstellar medium. It has recently been shown that the gamma-ray luminosity (Lγ) of star-forming galaxies is well correlated with their star formation rates (SFR). We investigated Fermi data of eight Galactic molecular clouds in the Gould belt and found that molecular clouds do not follow the Lγ −SFR correlation of star-forming galaxies. We also compared the scaling relations of gamma-ray luminosity, SFR, and the gas mass for molecular clouds and star-forming galaxies. Using a multiple-variable regression analysis, we found different dependences of gamma-ray emission on SFR or mass for molecular clouds and star-forming galaxies. This suggests that different mechanisms may govern the production of gamma-rays in these two types of sources. Specifically, the strong dependence on mass supports that gamma-ray emission of molecular clouds primarily comes from passive interaction by diffuse Galactic CRs, whereas the strong dependence on SFR supports that gamma-ray emission of star-forming galaxies originates from CRs that are accelerated by local active sources.

The Galactic Census of High- and Medium-mass Protostars. IV. Molecular Clump Radiative Transfer, Mass Distributions, Kinematics, and Dynamical Evolution

Abstract We present 12CO, 13CO, and C18O data as the next major release for the CHaMP project, an unbiased sample of Galactic molecular clouds in l = 280°–300°. From a radiative transfer analysis, we self-consistently compute 3D cubes of optical depth, excitation temperature, and column density for ∼300 massive clumps, and update the -dependent CO → H2 conversion law of Barnes et al. For N ∝ I p , we find p = 1.92 ± 0.05 for the velocity-resolved conversion law aggregated over all clumps. A practical, integrated conversion law is = (4.0 ± 0.3) × 1019 m−2 1.27±0.02, confirming an overall 2× higher total molecular mass for Milky Way clouds, compared to the standard X factor. We use these laws to compare the kinematics of clump interiors with their foreground 12CO envelopes, and find evidence that most clumps are not dynamically uniform: irregular portions seem to be either slowly accreting onto the interiors, or dispersing from them. We compute the spatially resolved mass accretion/dispersal rate across all clumps, and map the local flow timescale. While these flows are not clearly correlated with clump structures, the inferred accretion rate is a statistically strong function of the local mass surface density Σ, suggesting near-exponential growth or loss of mass over effective timescales ∼30–50 Myr. At high enough Σ, accretion dominates, suggesting gravity plays an important role in both processes. If confirmed by numerical simulations, this sedimentation picture would support arguments for long clump lifetimes mediated by pressure confinement, with a terminal crescendo of star formation, suggesting a resolution to the 40 year old puzzle of the dynamical state of molecular clouds and their low star formation efficiency.

The Arizona Radio Observatory CO Mapping Survey of Galactic Molecular Clouds. VI. The Cep OB3 Cloud (Cepheus B and C) in CO J = 2–1, 13CO J = 2–1, and CO J = 3–2

Abstract We present (1) new fully sampled maps of CO and J = 2–1 emission and CO J = 3–2 emission toward the molecular clouds Cep B and C, associated with the Cep OB3 association; (2) a map of extinction, A V , derived from IR colors of background stars; and (3) the distribution of young stellar objects (YSOs) over the same field as the molecular maps. An LTE analysis of the CO and maps yields the distribution of molecular column densities and temperatures. Substantial variations are evident across the clouds; smaller subregions show correlations between molecular properties and dust extinction, consistent with a picture of outer photodissociation regions with a layer of CO-dark molecular gas, a CO self-shielded interior, and an inner cold dense region where CO is largely depleted onto grains. Comparing the distribution of YSOs with molecular gas surface density shows a power-law relation very similar in slope to that for the giant molecular cloud associated with the H ii region Sh2-235 from a previous paper in this series that employed the same methodology. We note the presence of several compact, isolated CO emission sources in the J = 3–2 maps. The gas temperature and velocity dispersion yield a map of the sonic Mach number, which varies across the cloud but always exceeds unity, confirming the pervasiveness of supersonic turbulence over length scales ≳0.1 pc (the map resolution). We also compute a J = 2–1 CO X-factor that varies with position but is, on average, within 20% of the Galactic average derived from CO J = 1–0 observations.

Northern Galactic molecular cloud clumps in Hi-GAL: dense gas map and environmental trends

In the quest to understand high-mass star formation, it is necessary to understand that from which the high-mass stars will form -- the dense molecular gas clouds and clumps. The Herschel infrared GALactic plane survey (Hi-GAL) is a comprehensive survey of thermal dust emission that can be used to characterize the properties and Galactic distribution of molecular gas. We have analyzed the survey maps within the Galactic longitude range $10^\circ < \ell < 56^\circ$ and have transformed these 2D maps into a 3D dense gas map of the Galactic plane using distance probability density functions. This range corresponds to the extent of the Galactic Ring Survey (GRS), which provided the majority of our line-of-sight velocities, and thus kinematic distances. In this section of Hi-GAL, we identify 19,886 clumps, out of which 10,124 have, at minimum, a kinematic distance, and 5,405 have their distances well constrained through Bayesian techniques. Of those with well-constrained distances, clump masses tend to decrease with Galactocentric radius, whereas clump radii are independent of Galactocentric radius.